Coupled circuits



P. o. FARNHAM Jan. 30, 1934.

I COUPLED CIRCUITS Filed March 21,1932

I I I I I I I I I I I I I I l I YL Patented Jan. 30, 1934 UNITED STATESCOUPLED cmourrs Paul 0. Farnham, Boonton, N. J., assignor to RadioFrequency Laboratories, Incorporated, Boonton, N. J a corporation of NewJ ersey Application March 21,1932. Serial No. 600,358

10 Claims. 101113-44) This invention relates to coupled circuitsparticularly to methods of and circuit arrangements for varying thecoupling coeflicient between two circuits without varying the naturalresonant frequencies of the circuits and/or of the transmission line inwhich said circuits are incorporated. Transmission systems which includetwo circuits tuned to the same frequency and coupled with somewhat morethan critical coupling act as band pass filters as the resonance curveof the coupled circuits is not a single sharp peak but comprises arelatively fiat-topped curve resulting from the combination of twospaced resonance curves. The mean or natural" frequency of the coupledcircuits is the resonant frequency of the separate circuits and the twopeaks of the resonance curve are spaced from the natural resonantfrequency by an amount that varies with the coeflicient of coupling ofthe two circuits. With less than critical shape coupling, the resonancecurve has only one peak but the condition still obtains that the widthand shape of the curve depend upon the coeflicient of coupling;

An object of the present invention is to provide coupled circuitscapable of adjustment to vary the width of the resonance curve withoutaffecting the natural resonant frequency. A further object is to providetuned circuits that are coupled by a tuned impedance that may beeffectively removed from the network to change the coupling efficiency.More specifically, an object is to provide a system of two circuitstunable to the same frequency and which include coupling impedances thatare also tuned to the same fre quency, and switches for short-circuitingthe coupling impedances, thereby to alter the coupling coefficient ofthe two circuits without affecting the natural resonant frequency of thesystem.

These and other objects and advantages of the invention will be apparentfrom the following specification when taken with the accompanyingdrawing wherein:

Fig. 1 is a circuit diagram of an embodimentof the invention, and

Fig. 2 is a curve sheet illustrating theresonance curves obtainable witha coupling system such as shown in Fig. 1. I

In the drawing, the reference numerals 1, 1

. and 2, 2 identify, respectively, the input and the A capacitive thecoupling impedances of the circuits and, as

indicated by the broken line 4, these switches are preferablymechanically connected for simultaneous operation.

v When the switches are closed, the coupling between circuits I and IIis preferably very weak, approaching zero when C and M are zero, and theresonance curve has a single peak, as shown by curve A of Fig. 2. Forsimplicity of explanation, it will be assumed that both circuits aretuned to the same frequency. Under these conditions, the resonantfrequency ,f is given by the equations: a

In accordance with the invention, the values of the reactances of theshort-circuited coupling system are so related as to satisfy theequation:

Under these conditions, opening of the switches 3 to include thecoupling impedances in the circuits I and II will not alter the resonantfre- 100 than critical shape coupling, but it will be apparent that thesame the natural frequency absence of any change in i will attend theapplication of the invention to circuits in which the coupling is lessthan critical shape.

As indicated in Fig. 1, a magnetic coupling -M1 may be provided betweenthe i'nductances La, Lb of the coupling impedance. With magneticcoupling present, the total coupling controlled by switches 3 will be:

X,,,- :hwM we (4) To secure economy and tolerance, the sign of themagnetic coupling should be negative in order that the mutual inductivecoupling will aid the capacitive coupling Cm. Inductive coupling is notnecessary, however, so long as Equation (3) is satisfied.

With the coupling impedances La, Lb, Cm and, if employed, -M1, sorelated that the natural resonant frequency is not altered by theposition of switches 3, it will be apparent that the relativelypermanent couplings C and/ or M may be adjusted for less than criticalshape coupling, i. 'e. to give a comparatively sharp peak for theresonance curve. This adjustment is, of course, effected with theswitches 3 closed to short-circuit the complex coupling impedance. Theopening of switches 3 introduces a further coupling to broaden theresonance curve without changing the natural resonant frequency. Themagnitudes of the coupling impedance may be chosen to give any desiredbroadening of the resonance curve.

The invention is applicable not only to coupling systems in which theseparate circuits are tuned to the same frequency i, but also to systemsin which the individual circuits are tuned to different frequencies. Inaddition to its obvious use for varying the characteristics of a circuitduring the transmission of signals, the invention may be used tofacilitate the adjustment of the several elements of a circuit. Thealinement of the intermediate frequency circuits of a super heterodynereceiver is quite difficult when more than critical shape coupling isemployed in band pass circuits. Circuits designed as above described arereadily adjusted by first short-circuiting the coupling impedances andthentuncircuits I and II to the intermediate frequency f on which theoscillatortracking is based. The low coupling efficiency makes thisadjustment practical as the tuning of one circuit does not aifect theresonant frequency of the other circuit.

*hen the main circuits are to be tuned over a band of frequencies, thecoupling condenser Cm maybe mechanically connected to the tuningcondensers to maintain the relationships expressedby the above equationsthroughout the tuning, range of the circuits.

I claim:

1. The combination with two circuits, and a reactive coupling betweensaid circuits, of reactive means for varying the coefficient of couplingbetween said circuits while maintaining their natural resonant frequencyunchanged; said reactive means comprising serially connected ing each ofthe reactances coupling said circuits and resonating at the said naturalfrequency, and means for alternatively including said reactances in andexcluding the same from said circuits.

2. A pair of circuits each including main and auxiliary reactances, ineach circuit the main and auxiliary reactances being resonant at thesame frequency, a. coupling between the auxiliary reactances of saidcircuits, and means for changing the coefficient of coupling betweensaid circuits without altering the natural resonance frequency of saidcoupled circuits.

3. The invention as set forth in claim 2, wherein said means comprisesswitch means for including said auxiliary reactances in or excludingthesame from their respective circuits.

4. A pair of resonant circuits each including reactances coupled toprovide less than critical shape coupling, and means adapted to increasethe coupling to more than critical shape coupling, said means comprisingcoupling reactances and switch means for connecting said couplingreactances into the respective circuits, said coupling reactancesincluding a common portion and portions for introduction into therespective circuits, and each of said last portions resonating with saidcommon portion at the resonant frequency of the circuit into which therespective last portions may be introduced.

5 In a transmission circuit, the combination with a tuned circuit and asecond tuned circuit coupled thereto, of a coupling impedance includinga reactance of one type and a .pair of reactances of the opposite type,and switch means for connecting said coupling impedance into said twocircuits.

6. In a transmission circuit, the combination with a tuned circuit and asecond tuned circuit coupled thereto, of a coupling impedance includinga reactance of one type and. a pair of reactances of the opposite type,andswitch means for connecting said first reactance into both of saidcircuits and for connecting the reactances of said'pair into therespective circuits.

7. The combination with apair of circuits tuned to the same frequencyand coupled to resonate as a whole at that frequency, of means forvarying the coefficient of coupling between said circuits, said meanscomprising an impedance having two branches each resonating at the saidsame frequency.

8. A pair of tuned circuits each including inductance and a capacity, asecond capacity common to and coupling said circuits, of means forshort-'circuiting said common capacity and such portions of theinductanceof each of said circuits that the short-'circuited reactancesresonate at the natural resonance frequency of the coupled circuits.

9. The invention as set forth in claim 8, wherein the said portions ofsaid inductances are coupled magnetically.

10. The invention as set forth in claim 8, wherein the said portions ofsaid inductances are coupled magnetically to add to the capacitivecoupling of said common capacity.

PAUL O. FARNHAM.

